Processes for separating small particles contained in a suspension or slurry by size, wettability and other characteristics find application in various industries. The ability to make such separations is particularly desirable in paper making since the thickness and length of the pulp fibers are strongly related to the quality and characteristics of the paper produced from the fibers. Several specific potential uses in the paper industry for efficient fractionation processes have been identified.
A pulp slurry formed of reclaimed waste paper or paper board may be fractionated to remove clumps and particulate contaminants, and to separate fibers above and below a desired size. For example, such fractionation would allow the "linerboard" fibers in a slurry of waste corrugated fiberboard to be separated from the "medium fibers." Linerboard is mainly composed of softwood fibers of relatively large size (40-50 microns diameter, 3-5 mm length) whereas medium fibers are mainly hardwood fibers of smaller size (20-30 microns diameter, 1-3 mm length).
Fractionation also would allow a single fiber source, which ordinarily is a mix of fibers of various sizes, to be used optimally in the production of a desired multi-layered product. Each fraction, separated by fiber size, could be used to form a single layer which would have characteristics reflecting the size of the fibers in the layer. The layers of different fractions would then be combined to form a multi-layered product with qualities not possessed by a single layer product formed from the original fiber mix.
The separated pulp fractions also could be used alone to make single layer products having desired characteristics related to fiber size. In addition, some papermaking machines operate most efficiently with pulp having a particular fiber size range. Another potential application of pulp fractionation is the separation of a pulp stream into two or more fractions which can be beaten separately under optimium conditions and then recombined.
The fractionation apparatus disclosed in U.S. Pat. No. 4,427,541, the disclosure of which is incorporated herein by reference, has been shown to be highly effective in fractionating a slurry of fibers of varying diameter. This apparatus comprises a disk which is symmetrical about an axis of rotation with a face adapted to stabilize the film of the slurry deposited on the face, which terminates in a sharp, circular peripheral face edge. A descending rim or skirt extends from the face edge and terminates in a peripheral edge. This disk--which may have a planar face or an evenly concave or convex face--is rotated about a vertical axis. When the face and skirt of the disk are wettable and the particulate slurry is supplied to the face, coarse and/or poorly wettable particles are found to detach themselves from the flowing slurry film in a dewatered state and to move radially from the face edge and upper portion of the skirt in a relatively narrow band, while the fines are carried by the flowing liquid film over the surface of the skirt and disengaged, with the film, along the lower portion of the skirt or at the peripheral edge of the skirt. A separator wall may be positioned adjacent the rim to separate physically the two fractions of spray ejected from the disk, one carrying the coarse particles and the other the fines.
The chief limitation on the flow capacity of such an apparatus for fractionating a particle slurry is the extent to which the film stability can be maintained on the surface of the disk and break-up of the film prevented.